The present invention relates to an apparatus for reading from and/or writing to optical recording media which has a tilting-compensation device, and to a method for compensating for tilting.
An apparatus of this type has an optical scanner having a light source for generating a light pencil for scanning a recording medium, and a focusing means for focusing the light beam generated by the light source onto the recording medium. In apparatuses of this type, the situation arises where the focusing means exhibits tilting with respect to the optical axis of the light beam. Other optical elements of the scanner may also exhibit tilting. In order to compensate for tilting of this type, some apparatuses have a tilting-detection device and a tilting-compensation device, with the aid of which the tilting of the focusing means and/or of other optical elements is compensated for. The compensation of the tilting has the effect of reducing the error rate when reading from and/or writing to the optical recording medium.
An apparatus of this type with a tilting-compensation device is disclosed in U.S. Pat. No. 5,034,939. In this apparatus, the error rate of a data signal read from the recording medium is utilized for the purpose of compensating for tilting. In this case, a minimum error rate corresponds to optimum correction of tilting.
This known apparatus may be regarded as having the disadvantage that, in order to be able to carry out compensation of tilting, first of all a recording medium has to be read. This has the disadvantage that erroneously read data occur with increased frequency until the compensation of the tilting has been concluded, and this has an adverse effect for example on the reproduction of the data. Under certain circumstances, a relatively large degree of tilting means that it is not even possible to read from the recording medium in the first place. A further disadvantage may be seen in the fact that all the functions of the apparatus which are required for playback or reproduction of the recording medium are necessary for compensating for tilting. Therefore, the known method is also less suitable for tilting compensation that is to be carried out during the production process.
The object of the present invention is to propose an apparatus and a corresponding method which enables optimum compensation of tilting using as few apparatus functions as possible.
This object is achieved according to the invention, in an apparatus for reading from and/or writing to optical recording media, by virtue of the fact that the tilting-detection device has an optical grating which can be introduced into the beam path of the light source at the location of the recording medium, a detector element, a control device for switching on the light source and for moving the focusing means, and a control device for evaluating the signal of the detector element and for driving the tilting-compensation device for the purpose of varying and minimizing the tilting. This has the advantage that the apparatus according to the invention is able to implement compensation of tilting even without the presence of an optical recording medium and/or without tracking along the tracks of such a recording medium, with just a small number of necessary elements. Merely the contrast of the light coming from the grating is evaluated in this case in the focused state and in the defocused state. By way of example, the compensation of tilting for the apparatus according to the invention is possible as early as during the production of the said apparatus, the functionality of light source and focusing means being all that is required. In an advantageous manner, their functionality is tested at the same time as the compensation of tilting or with little time between them, for example in a test cycle. However, it is possible to set the compensation of tilting permanently, that is to say to dispense with later variation of the compensation of tilting. Another possibility is to carry out compensation of tilting for example during the operation of the apparatus, after the insertion of a new recording medium, before focusing has even been effected. This compensation of tilting is advantageously effected in a manner such that it is coupled with the focusing operation or other setting operations to be carried out before the actual read-out or writing mode is reached. The optical grating is advantageously a separate element, but it may also be formed by the recording medium. According to the invention, the detector element may be arranged both in the transmission position and in the reflection position with respect to the optical grating. It is advantageously a cost-effective detector of simple construction whose detector area can be designed to be relatively large. According to another advantageous variant, the detector element is a detector element which is arranged on the scanner and, during operation, is designed for example for reading out the data or as part of a regulating circuit.
According to the invention, the apparatus has a positioning device for positioning the optical grating parallel to a reference area of the apparatus. This has the advantage that the grating can be ideally positioned by means of the positioning device, with the result that the derived compensation of tilting leads to minimum tilting. The reference area is advantageously formed by guide elements of the scanner which enable the latter to be displaced radially with respect to the recording medium. If the grating is formed by the recording medium itself, then the positioning device is advantageously integrated in the customary holding mechanism for the recording medium. If a separate component is used as the grating, both the use of a separate positioning device and a positioning device which is coupled to the holding mechanism for the recording medium or is integrated therein are advantageously possible.
The apparatus according to the invention advantageously has a drive device for moving the optical grating parallel to one reference area of the apparatus. This has the advantage of ensuring that light coming from the grating can always be detected since an unfavourable relative position of grating and light source with respect to one another, which can occur randomly, is changed by the movement of the optical grating. A reciprocating movement of the grating with a small excursion is preferably provided in this case. As an alternative to this, it is provided that the focusing means and/or the scanner are moved relative to the optical grating. This also precludes the situation where grating and light source are in an unfavourable position relative to one another. In this case, use is preferably made of a drive which is present in any case for scanner and/or focusing means.
According to the invention, the optical grating has optical properties corresponding to the optical recording medium, such as thickness, refractive index, line width, line spacing or the like. This has the advantage that corruption of the measurement result and hence of the correction of tilting on account of different optical properties does not occur. In the case of an apparatus which is able to read from and/or write to recording media having greatly different optical properties, it is advantageous to use different gratings with corresponding properties.
The grating constant d of the optical grating, the wavelength xcex of the light source and the numerical aperture NA of the focusing means are advantageously related to one another in the relationship d=k*xcex/NA, where the proportionality factor k assumes a value of k=1,2xc2x110%. This has the advantage that the contrast between focused and non-focused states is greatest for this ratio, as a result of which the best possible compensation of tilting can be obtained. It is understood that deviations which are larger than those specified also lie within the scope of the invention, in which case, however, the requirements made of detector and/or evaluation device for the detection signal are then increased.
A method, according to the invention, for compensating for tilting is specified in the independent method claim. The method according to the invention has the advantage that merely the function of light source and focusing means is necessary, yet optimum compensation of tilting is nevertheless made possible. The order of the individual steps of the method can be modified without departing from the scope of the invention.
The invention provides for the grating to be oriented in the normal plane to the desired direction of the optical axis. This has the advantage that the compensation of tilting is optimized by optimum orientation.
The variation, provided according to the invention, of the position of the grating in the grating plane in a direction crossing the grating lines has the advantage of avoiding an unfavourable positionxe2x80x94which may occur randomlyxe2x80x94of the grating relative to the light pencil generated by the light source. In this case, too, it is advantageously possible to correspondingly change the relative position of grating and light source and/or of grating and focusing means.
According to the invention, provision is made, after an initial variation of the tilting, for rotating the grating and for varying the tilting again. In this case, each rotation by an angle greater than zero is expedient; it is advantageous to effect the rotation by an angle of 90xc2x0. The influence, on the detected signal, of tilting that cannot yet be compensated for in the course of the first variation process is at a maximum in this orientation of the grating; optimum compensation of tilting is thus possible. When a two-dimensional grating is used, it is also expedient to rotate the grating for the purpose of redundancy checking, in which case a value for the angle of rotation which lies between zero and 90xc2x0 is then advantageously used.
It is understood that the invention is not restricted to the features described, but also includes developments within the abilities of a person skilled in the art. Advantageous refinements of the invention are contained in the following description of preferred exemplary embodiments.